全文获取类型
收费全文 | 12789篇 |
免费 | 655篇 |
国内免费 | 63篇 |
专业分类
化学 | 9268篇 |
晶体学 | 42篇 |
力学 | 303篇 |
数学 | 2000篇 |
物理学 | 1894篇 |
出版年
2023年 | 95篇 |
2022年 | 143篇 |
2021年 | 204篇 |
2020年 | 297篇 |
2019年 | 312篇 |
2018年 | 147篇 |
2017年 | 170篇 |
2016年 | 455篇 |
2015年 | 484篇 |
2014年 | 496篇 |
2013年 | 674篇 |
2012年 | 951篇 |
2011年 | 956篇 |
2010年 | 593篇 |
2009年 | 516篇 |
2008年 | 767篇 |
2007年 | 737篇 |
2006年 | 680篇 |
2005年 | 669篇 |
2004年 | 527篇 |
2003年 | 424篇 |
2002年 | 413篇 |
2001年 | 200篇 |
2000年 | 161篇 |
1999年 | 156篇 |
1998年 | 168篇 |
1997年 | 162篇 |
1996年 | 138篇 |
1995年 | 110篇 |
1994年 | 103篇 |
1993年 | 110篇 |
1992年 | 87篇 |
1991年 | 74篇 |
1990年 | 67篇 |
1989年 | 88篇 |
1988年 | 62篇 |
1987年 | 46篇 |
1986年 | 52篇 |
1985年 | 70篇 |
1984年 | 65篇 |
1983年 | 56篇 |
1982年 | 65篇 |
1981年 | 67篇 |
1980年 | 56篇 |
1979年 | 55篇 |
1978年 | 56篇 |
1977年 | 56篇 |
1976年 | 59篇 |
1975年 | 52篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Christian K. Jørgensen 《Chemical physics letters》1982,89(6):455-458
Recent relativistic calculations on the uranyl ion suggest that the low wave numbers of the first electron transfer bands are due to a bonding 5f-like orbital containing the two loosest bound electrons. Comparison with UF6 makes it more likely that if indeed σu (and not πu) is the highest occupied MO, it is rather due to “pushing from below” by U 6p (like N 2s in N2). 相似文献
152.
1,3,4-Oxadiazoles can be rapidly and efficiently synthesized from a variety of carboxylic acids and acid hydrazides in one simple step. The use of commercially available PS-PPh3 resin combined with microwave heating delivered the product 1,3,4-oxadiazoles in high yields and purities. 相似文献
153.
Russier-Antoine I Jonin C Nappa J Bénichou E Brevet PF 《The Journal of chemical physics》2004,120(22):10748-10752
The wavelength dependence of the quadratic hyperpolarizability of 11 nm diam gold nanoparticles, is reported as measured by hyper Rayleigh scattering. An important photoluminescence background underlying the hyper Rayleigh signal is observed, a contribution attributed to radiative electron-hole recombinations following multiphoton excitation favored by adsorbed organic compound like citrate on the surface of the nanoparticles. The absolute value of the quadratic hyperpolarizability of gold spherical nanoparticles is determined and a strong enhancement is observed for harmonic frequencies in resonance with the dipolar surface plasmon excitation. No contribution of the interband transition is observed. The absolute values reported, beta(C)=5.1x10(-26) esu at the second harmonic energy 2.39 eV, have been measured with femtosecond long laser pulse, and are 1 order of magnitude weaker that the one previously reported with nanosecond long pulses. This difference can be related to similar measurements performed on the second order hyperpolarizability of gold nanoparticles and may be attributed to different electronic relaxation regimes. Finally, the spectrum of the quadratic hyperpolarizability is compared to the theoretically expected one. 相似文献
154.
155.
M. M. Abd-Elmeguid Ch. Sauer U. Köbler W. Zinn 《Zeitschrift für Physik B Condensed Matter》1985,60(2-4):239-248
X-ray diffraction,151Eu Mössbauer effect (ME) and magnetic susceptibility measurements have been performed to investigate the valence behavior and the magnetic interactions of the Eu ions in the intermetallic series Eu(Pd1–x
Au
x
)2Si2 (0x1) as a Forx>0.4 all experimental techniques are in agreement with each other and suggest a divalent 4f
7 ground state of the Eu ion. Belowx=0.4 the different methods lead to different results: while the lattice parameters and the ME isomer shift suggest an instable behavior of the valence the magnetic susceptibility proves a pure divalent 4f
7 ground state. These differences can be explained by assuming a partial extension of the 4f-shell radius such that the localized character of the 4f-electrons is preserved.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday 相似文献
156.
Salama Tarek A. El-Ahl Abdel-Aziz S. Khalil Abdel-Galil M. Girges Margret M. Lackner Bernd Steindl Christian Elmorsy Saad S. 《Monatshefte für Chemie / Chemical Monthly》2003,134(9):1241-1252
Summary. Several new 1-aryl-, aralkyl-, and heteroaryl-5-(4-phenylbuta-1,3-dienyl)tetrazole derivatives and annulated tetrazole derivatives were efficiently and regiospecifically prepared in nearly quantitative yield via a facile one step reaction of dienones with a combination of tetrachlorosilane and sodium azide in acetonitrile under mild conditions. A complete structure assignment of three representative examples of the tetrazoles was achieved by advanced 2D NMR measurements including COSY, TOCSY, HSQC, HMBC, NOESY, and ROESY experiments.Received March 17, 2003; accepted March 18, 2003
Published online July 28, 2003 相似文献
157.
Hedi Hannachi Naoual Anoune Christian Arnaud Pierre Lant ri R mi Longeray Henry Chermette 《Journal of Molecular Structure》1998,434(1-3):183-191
The Baeyer-Villiger reaction of polymethoxybenzaldehydes with peroxysuccinic acid (PSA) is studied theoretically. The semi-empirical SCF-MO/PM3 method has been used to study the reaction and to calculate the energy profiles. In addition, heat of formation computations and bond order analyses of the transition species along the reaction coordinates have been performed and are interpreted qualitatively in terms of electron flow through the system during aryl migration. Our results support the hypothesis that the second step is a concerted reaction. Our data support a definite effect of the substituent on the aryl migration. The theoretical predictions are in good agreement with experimental results. 相似文献
158.
159.
Bernd?Lackner Yulita?Popova Christoph?Etzlstorfer Andrija A.?Smelcerovic Christian W.?Klampfl Heinz?FalkEmail author 《Monatshefte für Chemie / Chemical Monthly》2005,136(5):777-793
Summary. The syntheses of the two heterocyclically substituted title hypericin derivatives were achieved starting either from 6-benzothiazolyl-tri-O-methyl-6-demethylemodin or 6-benzoxazolyl-tri-O-methyl-6-demethylemodin. The use of microwave assisted synthesis for the preparation of these anthraquinone synthons and the chemical as well as photochemical properties of the corresponding unique hypericin derivatives, which might constitute new photodynamic therapy agents, are reported. The tautomeric and stereochemical aspects of these hypericin derivatives were investigated by means of semiempirical calculations (AM1). 相似文献
160.
Alexander Chernega Dirk. L. Elend Christian A.P. Smethurst Paul M. Roberts Andrew D. Smith G. Darren Smyth 《Tetrahedron》2007,63(30):7036-7046
A three-step protocol for the asymmetric synthesis of a range of β-substituted Baylis-Hillman products has been developed. This procedure involves the diastereoselective conjugate addition of lithium (R)-N-methyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester to generate an N-protected β-amino ester in high de. Subsequent asymmetric aldol reaction via deprotonation with LDA, transmetallation with B(OMe)3 and addition of an aldehyde gives a range of syn-aldol products in moderate to high de. Purification of the syn-aldol products to homogeneity followed by tandem N-oxidation and Cope elimination gives the desired β-substituted Baylis-Hillman products in good yield and high de and ee. 相似文献